1. Field of the Invention:
This invention relates to a fluorescent body. More particularly, it is a fluorescent body having a novel luminescent mechanism which can advantageously be used for making various kinds of electroluminescent (EL) elements.
2. Description of the Prior Art:
There are known various kinds of fluorescent bodies which are used for making EL elements. They mainly comprise a luminescent material, such as ZnS, and contain Cu, Cl, I, Al, Mn, etc. as an activator. Their combinations include (ZnS:Cu, Cl), (ZnS:Cu, I), (ZnS:Cu, Al) and (ZnS: Cu, Mn).
When a voltage is applied to any such fluorescent body to excite its activator, it is necessary to apply a voltage of at least 10.sup.6 V/cm in a direct electric field. The electric field which is usually applied to an EL element, however, has a strength of, say, 1 to 3.times.10.sup.4 V/cm, and this strength is actually high enough to excite the activator. The fluorescent body of the type as hereinabove described contains a greater amount of copper than any ordinary type of fluorescent body used for exciting cathode rays, etc., so that Cu.sub.x S may be precipitated in the crystal defect or grain boundary of the luminescent material (ZnS) and form an interface with ZnS defining an energy barrier which enables the partial creation of an electric field having a strength of at least 10.sup.6 V/cm. This is the luminescent mechanism of the fluorescent body.
The fluorescent body is said to be a polycrystalline substance 30 having a grain diameter of several tens of microns and containing Cu.sub.x S 31, as shown in FIG. 6. When a dispersion type EL element is manufactured, a fluorescent body having a grain diameter of, say, 20 to 30 microns is, for example, mixed with a binder. The mixture is applied to a transparent electrode on a sheet of glass or a film to form a luminescent layer thereon and a back electrode is attached thereto, while an insulating layer is provided therebetween.
The luminescent layer in the EL element as hereinabove described has a thickness of, say, 50 to 100 microns, as the fluorescent body has a grain diameter of several tens of microns. If a fluorescent body having a smaller grain size is used, it is possible to form a luminescent layer having a smaller thickness and a higher degree of uniformity and density which make it more suitable for practical application.
A number of methods are, therefore, employed for producing a fluorescent body having a smaller grain size. They include etching and mechanical crushing or classification. However, the conventional products of these methods have usually had a number of drawbacks including low luminance. As the fluorescent body has a luminescent mechanism introduced in the crystal defect or grain boundary of the luminescent material, its luminance is greatly dependent on their size or number. If the crystal defects, etc. are increased to increase the luminescent mechanism, the fluorescent body has only a limited degree of crystallinity. Any of the conventional methods as hereinabove mentioned can only reduce the grain size of the fluorescent body to a level of, say, three microns. This level of reduction even does harm to the crystal, rather than being effective. Therefore, there has hitherto been no alternative but to employ a fluorescent body having a grain diameter of several tens of microns in order to obtain a practical compromise between its crystallinity and the performance of the luminescent mechanism. It has naturally been impossible to form a luminescent layer having a satisfactorily small thickness and a satisfactorily high degree of uniformity and density. It has, therefore, been difficult to produce any fluorescent body that enables a high luminescent efficiency, being driven by a low voltage and having a high degree of luminance.